Fracturing of rocks in Bavarian geothermal reservoirs – Experimental and numerical investigation of borehole stability

Abstract

<p>As one of the most promising settings for deep geothermal energy, the North Alpine Foreland Basin in Bavaria (SE Germany) is currently a location of ongoing planning, development, and operation of hydrothermal energy projects. The high risks associated with the more than 3000 m deep wells, drilled into the reservoir still limits an accelerated exploration and usage of the reservoir’s potential. The heterogeneities of rock mass and thus uncertain subsurface conditions threaten the wellbore integrity and lower the predictability of hydraulic reservoir stimulation and its success. Characterizing the fracturing properties of in situ samples and analogue rocks, extracted from nearby quarries, aims to increase certainty in expected fracturing processes.  </p><p>To determine the fracture toughness and fracture energy in tensile (mode I) and shear mode (mode II) two experimental setups are used. With the double edge-notched Brazilian disk (DNBD) test the mode II fracture toughness is measured by inducing shear failure through uniaxial compression of a bi-notched Brazilian disk. The semi-circular bend (SCB) test applies tensile forces on a notched semi-cylindrical disk by bending the sample around three roller supports and produces mode II fracture. A high-speed camera records the test procedure for further characterizing the fracture propagation and the influence of inhomogeneities.</p><p>Together with elastic and strength properties of the analog rocks, extracted from further rock mechanical tests, the fracture energy values serve as input for a numerical study of borehole stability. By using the hybrid FEM-DEM method, complex 2D and 3D borehole models are computed without losing the relation to realistic processes. In multiple scenarios, that are representative of the geothermal projects in the North Alpine Foreland Basin, the rock mass behavior during borehole excavation is simulated with the Irazu software. Additionally, pre-existing fracture networks, in situ stresses and pore-fluid pressure are integrated, which influence the resulting fracture pattern and fracturing degree. The coupled mechanical-hydraulical model will later be extended to additionally incorporate thermal effects.</p><p>When considered during future drill operations these results can help to lower the economic risk that is still associated with deep geothermal operation and consequently increase the development speed to a sustainable heat energy provision in SE Germany.</p><p>This work is part of the Geothermal-Alliance Bavaria and funded by the Bavarian State Ministry of Science and Arts (StMWK).</p><p> </p><p>Keywords: numerical FDEM simulation; laboratory testing; fracture energy; geothermal wellbores; North Alpine Foreland Basin, SE Germany</p>